Servo control systems that maintain the position of read/write heads relative to tracks on discs in disc drives, for example, are well known. To provide proper position control, such servo systems generate position error signals (PES) indicative of the position of the heads from servo information that is written to the discs during the manufacturing of the disc drive. In response to the detected position, the servo system outputs current to an actuator motor (such as a voice coil motor, or VCM) utilized to pivot an actuator assembly that moves the heads across the disc surfaces.
It is a continuing trend in the disc drive industry to provide successive generations of disc drive products with ever increasing data storage capacities and data transfer rates. Because the amount of disc surface area available for the recording of data remains substantially constant (or even decreases as disc drive form factors become smaller), substantial advancements in areal recording densities, both in terms of the number of bits that can be recorded on each track as well as the number of tracks on each disc, are continually being made in order to facilitate such increases in data capacity.
The servo information used to define the tracks is written during disc drive manufacturing using a highly precise servo track writer. While the tracks are intended to be concentric, uncontrolled factors such as bearing tolerances, spindle resonance modes, misalignment of the discs and the like tend to introduce errors in the location of the servo information. Each track is thus typically not perfectly concentric, but rather exhibits certain random, repeatable variations which are sometimes referred to as written-in repeatable runout, or WI-RRO. In addition to the WI-RRO, repeatable disturbance in a disc drive also occurs due to an unbalanced spindle, for example. The WI-RRO and other repeatable disturbances are collectively referred to as repeatable runout (RRO). RRO appears as a component of the PES. Another component of the PES called non-repeatable runout (NRRO) occurs due to non-repeatable disturbances such as resonance modes, disc flutter, windage, disc vibrations, etc. RRO has a constant period determined by the spindle speed of the disc drive, and NRRO is random, but with consistent probability distributions on different tracks of discs of the disc drive.
While RRO has previously had a minimal impact upon the operation of the disc drive servo system, RRO has an increasingly adverse affect as higher track densities are achieved. Particularly, RRO can ultimately lead to an upper limit on achievable track densities, as RRO cuts into the available track misalignment budget and reduces the range over which the servo system can provide stable servo control. Therefore, relatively accurate RRO measurements need to be carried out for PES analysis during the manufacture of disc drives with high track densities.
Techniques for measuring or estimating RRO usually determine RRO values for sectors of each track by averaging the PES for sectors of each track over several disc revolutions. Since NRRO is also present in the PES, and since the NRRO may be relatively large before the head properly settles over a track, such an averaging technique may produce inaccurate results.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.